Zeolite catalysts have become widely used in the processing of petroleum and in the production of various petrochemicals. Reactions such as cracking, hydrocracking, catalytic dewaxing, alkylation, dealkylation, transalkylation, isomerization, polymerization, addition, disproportionation and other acid catalyzed reactions may be performed with the aid of these catalysts. Both natural and synthetic zeolites are known to be active for these reactions.
The common crystalline zeolite catalysts are the aluminosilicates such as zeolites A, X, Y and mordenite. Structurally, each such material can be described as a robust three dimensional framework of SiO4 and AlO4 tetrahedra that are crosslinked by the sharing of oxygen atoms whereby the ratio of total aluminum and silicon atoms to oxygen is 1:2. These structures (as well as other crystalline zeolites of catalytic usefulness) are porous, and permit access of reactant molecules to the interior of the crystal through windows formed of eight-membered rings (small pore) or of twelve-membered rings (large pore). The electrovalence of the aluminum that is tetrahedrally contained in the robust framework is balanced by the inclusion of cations in the channels (pores) of the crystal.
An "oxide" empirical formula that has been used to describe the above class of crystalline zeolites is EQU M.sub.2/n O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O
wherein M is a cation with valence n, x has a value of from 2 to 10, and y has a value which depends on the pore volume of the particular crystal structure under discussion. The empirical oxide formula may be rewritten as a general "structural" formula EQU M.sub.2/n O[(AlO.sub.2).w(SiO.sub.2)]yH.sub.2 O
wherein M and y are defined as above, and wherein w has a value from 1 to 5. In this representation, the composition of the robust framework is contained within the square brackets, and the material (cations and water) contained in the channels is shown outside the square brackets. One skilled in the art will recognize that x in the empirical oxide formula represents the mole ratio of silica to alumina in the robust framework of a crystalline zeolite, and shall be referred to herein simply by the expression in common usage, i.e. "the silica to alumina ratio". (See Zeolite Molecular Sieves, Donald W. Breck, chapter one, John Wiley and Sons, New York, N.Y., 1974, which is incorporated herein by reference as background material).
With few exceptions (such as with Zeolite A wherein x=2), there are fewer alumina tetrahedra than silica tetrahedra in the robust frameworks of the crystalline zeolites. Thus, in general, aluminum represents the minor tetrahedrally coordinated constituent of the robust frameworks of the common zeolites found in nature or prepared by the usual synthetic methods that employ only inorganic reagents.
For the above common zeolite compositions, wherein x has a value of 2 to 10, it is known that the ion exchange capacity measured in conventional fashion is directly proportional to the amount of the minor constituent in the robust framework, provided that the exchanging cations are not so large as to be excluded by the pores. If the zeolite is exchanged with ammonium ions and calcined to convert it to the hydrogen form, it acquires a large catalytic activity measured by the alpha activity test for cracking n-hexane, which test is more fully described below; and the ammonium form itself desorbs ammonia at high temperature in a characteristic fashion.
It is generally recognized that the composition of the robust framework of the synthetic common zeolites, wherein x=2 to 10, may be varied within relatively narrow limits by changing the proportion of reactants, e.g. by increasing the concentration of the silica relative to the alumina in the zeolite forming mixture. However, definite limits in the maximum obtainable silica to alumina ratio are observed. For example, synthetic faujasites having a silica to alumina ratio of about 2.5 to 5.6 can be obtained by changing said relative proportions. However, if the silica proportion is increased above the level which produces the 5.6 ratio, no commensurate increase in the silica to alumina ratio of the crystallized synthetic faujasite is observed. Thus, the silica to alumina ratio of about 5.6 must be considered an upper limit for synthetic faujasite in a preparative process using conventional reagents. Corresponding upper limits in the silica to alumina ratio of mordenite and erionite via the synthetic pathway are also observed.
It is sometimes desirable to obtain a particular zeolite, for any of several reasons, with a higher or a lower silica to alumina ratio than is available by direct synthesis. With ion-exchange applications, for example and for catalytic reactions such as hydrocracking which require high acidity catalysts, low silica to alumina ratios are favorable. For structural stability to heat and steam, or high-temperature xylene isomerization, high silica to alumina ratios are required. U.S. Pat. No. 4,273,753 to Chang and the references contained therein describe several methods for removing some of the aluminum from the framework by the use of aggressive treatments such as steaming, contact with chelating agents, etc., thereby increasing the silica to alumina ratio of a crystal. However, no generally useful method appears to have been described for increasing the alumina content (decreasing the silica to alumina ratio) of a zeolite crystal. Thus, although it is relatively easy to reversibly alter the composition of the materials (cations and water) contained within the channels of the crystalline zeolites, no generally useful method is known for reversibly altering the content of the minor tetrahedrally coordinated constituent in the robust framework.
A class of synthetic high silica content crystalline zeolites wherein x is at least 12, has recently been discovered. In general, such zeolites are made from a forming solution which contains an organic template. Unlike the common synthetic zeolites, these high-silica content zeolites appear to have no natural counterpart. Members of this new class of zeolites have many advantageous properties, which properties generally include a high degree of structural stability. They are used or have been proposed for use in various processes, especially catalytic processes. Known materials of this type include, for example, zeolite Beta (U.S. Pat. No. 3,308,069); ZSM-5 (U.S. Pat. No. 3,702,886); ZSM-11 (U.S. Pat. No. 3,709,979); and ZSM-12 (U.S. Pat. No. 3,832,449).
The SiO.sub.2 /Al.sub.2 O.sub.3 ratio of a given molecular sieve is often variable. For example, zeolite X can be synthesized with SiO.sub.2 /Al.sub.2 O.sub.3 ratios of from 2 to 3; zeolite Y, from 3 to about 6. In some zeolites, the upper limit of the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is unbounded. ZSM-5 is one such example wherein the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is at least 5 and up to the limits of present analytical measurement techniques. U.S. Pat. No. 3,941,871 (RE. 29,948) discloses a porous crystalline silicate made from a reaction mixture containing no deliberately added alumina in the recipe and exhibiting the X-ray diffraction pattern characteristic of ZSM-5. U.S. Pat. Nos. 4,061,724; 4,073,865; and 4,104,294 describe crystalline silicate of varying alumina and metal content.
In some instances, incorporating an oxide of boron in the forming solution of a high silica content zeolite results in a zeolite that contains framework boron.